A method is disclosed in U.S. Pat. No. 3,928,537 for removing sulfur dioxide in the form of gypsum from an exhaust gas, such as a combustion exhaust gas by the treatment of the exhaust gas with an aqueous solution containing an organic acid alkali salt. According to U.S. Pat. No. 3,928,537, the removal of sulfur dioxide in the form of gypsum from the exhaust gas is accomplished by bringing the exhaust gas into contact with an aqueous solution containing an organic acid alkali salt to absorb the sulfur dioxide in the form of alkali sulfite and alkali sulfate, blowing oxygen or air into the aqueous solution containing the absorbed sulfur dioxide, thereby oxidizing the alkali sulfite into the corresponding alkali sulfate, adding thereto calcium carbonate or calcium hydroxide, thereby converting the alkali sulfate into calcium sulfate (gypsum) and separating the calcium sulfate, and recirculating the solution for contact with the incoming exhaust gas. The reaction mechanism involved in this method is expressed by the following reaction formulas (1) through (4) EQU 2RCOOM + SO.sub.2 + H.sub.2 O .fwdarw. 2RCOOH + M.sub.2 SO.sub.3 ( 1) EQU m.sub.2 so.sub.3 + 1/2 o.sub.2 .fwdarw. m.sub.2 so.sub.4 ( 2) EQU 2rcooh + caCO.sub.3 .fwdarw. (RCOO).sub.2 Ca + CO.sub.2 + H.sub.2 O (3) EQU m.sub.2 so.sub.4 + (rcoo).sub.2 ca .fwdarw. CaSO.sub.4 + 2RCOOM (4)
in the preceding formulas, RCOOM represents an organic acid alkali salt, RCOO an organic acid group and M an alkali metal or NH.sub.4, respectively.
In the method described above, however, the solution remaining after the separation of calcium sulfate (which is an aqueous solution containing an orgaic acid alkali salt as indicated by Formula (4) given above) still has calcium sulfate dissolved therein in an amount corresponding to its solubility. When the recirculated solution still containing the dissolved calcium sulfate is returned to the absorber, the dissolved calcium sulfate crystallizes out in the form of calcium sulfite crystals or gypsum crystals and deposits within the absorption column and other process units. In the recycled solution, the alkali sulfite formed in consequence of the absorption of sulfur dioxide and the calcium sulfate dissolved in the solution react as indicated by formula (5), with the result that calcium sulfite, a compound far less soluble than calcium sulfate, is formed and crystallized. EQU CaSO.sub.4 + M.sub.2 SO.sub.3 .fwdarw. CaSO.sub.3 + M.sub.2 SO.sub.4 (5)
further, in the cyclic use of the solution, the absorption of sulfur dioxide results in a lowered pH value and in a consequent decline of the solubility of gypsum and, at the same time, part of the alkali sulfite formed in consequence of the absorption of sulfur dioxide is oxidized by the oxygen contained in the exhaust gas and consequently converted into a corresponding alkali sulfate to increase the sulfate ion concentration in the solution, resulting in the so-called ionic product effect (common-ion effect). Thus, the crystallization of dissolved gypsum occurs in the solution. If, for example, the exhaust gas is brought into contact with an aqueous solution containing 12 percent by weight of sodium acetate, 0.3 percent by weight of acetic acid and 1 percent by weight of sodium sulfite and, as a result of continued contact, the aqueous solution comes to contain 10 percent by weight of sodium acetate and 1.8 percent by weight of acetic acid and consequently shows a lowered pH value, the solubility of gypsum in the aqueous solution declines from the original level of about 0.7 percent to about 0.3 percent. When calcium sulfite crystals or gypsum crystals are allowed to crystallize out within the reaction system in this manner, these crystals deposit on the inner wall surfaces of the process equipment causing scaling.
Moreover, in the closed system as described above wherein the exhaust gas containing sulfur dioxide is treated with an aqueous solution containing an organic acid alkali salt, the aqueous solution contains chlorine ions which are introduced with the industrial service water used to form and maintain the aqueous solution. The calcium carbonate or calcium hydroxide used for treatment also usually contains chlorides. In the case where the exhaust gas happens to be a combustion exhaust gas derived from the combustion of a fuel containing chlorides such as coal, the combustion exhaust gas carries chlorides originating in the fuel. In the circumstance, chlorine ions accumulate in the solution being circulated through the system. Usually, the combustion exhaust gas from the combustion of coal contains 30 to 150 ppm of chlorine in the form of chlorides and the industrial service water contains 20 to 50 ppm of chlorine in the form of chlorides. In addition, the industrial grade calcium carbonate contains 40 to 100 ppm of chlorine in the form of chlorides. In the treatment of 1,000,000 Nm.sup.3 /hour of the combustion exhaust gas from the combustion of coal containing 1000 ppm of sulfur dioxide, for example, chlorine ions accumulate at the rate of several tens of kg per hour in the solution being circulated through the system. The chlorine ions which thus accumulate in the solution are crystallize out predominantly in the form of alkali chlorides. Simultaneously with or prior to the crystallization of such alkali chlorides, other salts such as alkali sulfate and the like may crystallize. The resultant crystals also deposit as scale on the inner wall surfaces of the reaction equipment. The chlorine ions accumulated in the solution circulated in the system not merely cause the formation of scale as described above but also cause corrosion of the reaction equipment.
As the scale forms on the inner wall surfaces of the reaction equipment as described above, the interior of the absorption column experiences an increased pressure loss and the interiors of the pipes become clogged, posing various problems in the operation, maintenance and control of the reaction system. The methods for the removal of the scale thus formed in the system include a chemical treatment whereby the scale is dissolved and purged by use of chemicals and a mechanical treatment whereby the scale is scraped off and washed out with water continuously or intermittently. The former treatment has the disadvantage that the operation of the reaction system must temporarily be suspended for the system to undergo the treatment or, if the operation is not desired to be interrupted by the treatment, the reaction system must be provided with extra standby units representing a heavy expense in terms of operation or equipment. In the latter treatment, thorough removal of the scale can not satisfactorily be obtained particularly when the reaction equipment is of a large capacity.